The present invention relates to an optical pickup which is indispensable part for an information reproducing apparatus (disc player) for playing a CD (compact disc) and a DVD (digital video disc).
A compact disc (hereinafter referred to as CD) comprises a plastic layer (having a thickness of 1.2 mm) on which audio or video signals are recorded in the form of numerous information pits, a metal reflecting layer formed on the plastic layer, and a protection layer formed on the reflecting layer.
A digital video disc (hereinafter referred to as DVD) comprises two information layers on each of which signals are recorded in the form of numerous information pits, each having a thickness of 0.6 mm. Such a DVD is capable of recording a much larger amount of information than a CD.
Although a CD and a DVD are different from each other in their specifications, a CD drive of a CD player has many similarities with a DVD drive of a DVD player. Thus, there has been suggested an optical disc player which has two optical pickups adapted to play a CD and a DVD, using only a single disc drive.
However, in the optical disc player designed to play a CD and a DVD, since there are two optical pickups, two sets of moving mechanisms are required to respectively move the two optical pickups in disc radial direction. As a result, it is difficult for a disc player to be made compact.
In view of the above, a more compact player has been demanded which has only a single optical pickup capable of playing not only a CD but also a DVD. For instance, the single optical pickup is made into two-focus pickup capable of emitting an optical beam for playing both a CD and a DVD (whose protection layers are different from each other in thickness). However, in a case where a two-focus optical pickup is employed in a disc player, if there is an inclining angle between the recording surface of an optical disc and the optical axis of the optical beam, a comatic aberration will occur in the optical beam when irradiating an optical disc. in order to correct such a comatic aberration, a pickup including an actuator is moved in disc radial direction or disc tangential direction, thereby eliminating the inclining angle between the recording surface of an optical disc and the optical axis of an optical beam.
On the other hand, since an optical beam for DVD and an optical beam for CD are caused to pass through an identical objective lens with different incident angles, an attempt to eliminate a comatic aberration occurring in one optical beam will undesirably promote a comatic aberration occurring in the other optical beam. Namely, it is difficult to eliminate two kinds of comatic aberration occurring in two optical beams.
In order to solve the above problem, at first the pickup body is caused to incline to some extent to eliminate a comatic aberration occurring in one optical beam. Then, the actuator of the pickup body is caused to incline to some extent in a direction in which a comatic aberration occurs in the other optical beam. Further, it is also possible to have the pickup body inclined to at the same time eliminate the above two kinds of comatic aberration occurring in the above two optical beams.
It is understood from the above explanation that, in order to correct two kinds of comatic aberration occurring in two optical beams, it is necessary to establish a construction where the pickup body of the two-focus pickup may be made inclined with respect to the recording surface of an optical disc. For instance, it is required that the pickup body be divided into a main pickup body and a sub-pickup body, both of which are pivotably connected with each other, such that the main pickup body may be made inclined so as to correct the above comatic aberration.
However, in a construction where the main pickup body and a sub-pickup body are pivotably connected with each other, a resonance phenomenon will occur in both the main pickup body and the sub-pickup body due to a vibration from the outside, resulting in a problem that information can only be reproduced with a deteriorated precision.